<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        .header {
            position: fixed;
            top: -80px;
            left: 0;
            width: 100%;
            height: 80px;
            background-color: purple;
            text-align: center;
            color: #fff;
            line-height: 80px;
            font-size: 30px;
            transition: all .3s;
        }

        .content {
            overflow: hidden;
            width: 1000px;
            height: 3000px;
            background-color: pink;
            margin: 0 auto;
        }

        .backtop {
            display: none;
            width: 50px;
            left: 50%;
            margin: 0 0 0 505px;
            position: fixed;
            bottom: 60px;
            z-index: 100;
        }

        .backtop a {
            height: 50px;
            width: 50px;
            background: url(./images/bg2.png) 0 -600px no-repeat;
            opacity: 0.35;
            overflow: hidden;
            display: block;
            text-indent: -999em;
            cursor: pointer;
        }

        .sk {
            width: 300px;
            height: 300px;
            background-color: skyblue;
            margin-top: 500px;
        }
    </style>
</head>

<body>
    <div class="header">我是顶部导航栏</div>
    <div class="content">我是div父级元素
        <div class="sk">秒杀模块我是子元素</div>
    </div>

    <!-- 返回顶部 -->
    <div class="backtop">
        <img src="./images/close2.png" alt="">
        <a href="javascript:;"></a>
    </div>
    <script>
        const sk = document.querySelector('.sk')
        const header = document.querySelector('.header')
        const backtop = document.querySelector('.backtop')
        // 1. 页面滚动事件
        window.addEventListener('scroll', function () {
            // 当页面滚动到 秒杀模块的时候，就改变 头部的 top值
            // 页面被卷去的头部 >=  秒杀模块的位置 offsetTop
            const n = document.documentElement.scrollTop
            // if (n >= sk.offsetTop) {
            //     header.style.top = 0
            // } else {
            //     header.style.top = '-80px'
            // }
            header.style.top = n >= sk.offsetTop ? 0 : '-80px'
            backtop.style.display = n >= sk.offsetTop ? 'block' : 'none'
            backtop.addEventListener('click', function () {
                // window.scrollTo(0, 0)
                document.documentElement.scrollTop =1
            })
        })
    </script>
</body>

</html>